As is known, pseudolites, a contraction of pseudo-satellites, are devices operating according to the same principles as satellites belonging to constellations of satellites used in the context of global navigation satellite systems, known by the acronym GNSS, for Global Navigation Satellite System, such as the GPS system, for Global Positioning System, or the Galileo system. Unlike satellites, pseudolites are deployed on the ground. They can be distributed in a building and, in general, in “confined” areas.
In a pseudolite positioning system, said pseudolites transmit positioning signals of which the format is identical or similar to that of the messages transmitted by the satellites of a satellite navigation system. These positioning signals are notably characterized by their time desynchronization and by their frequency desynchronization, or apparent Doppler frequency, at the receiver. In this context, each pseudolite is usually allocated an identifier of the same family as that of a satellite. In the context of constellations of satellites, these identifiers are called spreading codes as is known to those skilled in the art.
The range of the signals transmitted by the pseudolites is variable; it depends on their power and on their use. Objects fitted with suitable receivers can acquire these positioning signals. As for a conventional satellite navigation system, a computation of pseudo-distances between said receiver and the pseudolites of which it has acquired the signals, followed by a computation of position by triangulation, make it possible to determine the location of the receiver. The principle of positioning by triangulation is known: it involves determining the position of a receiver as being at the intersection of spheres of centre the transmitters and of radius the distance between receiver and transmitters. The computations can be made on board, by the object itself, or remotely by a computer.
As has been seen, the pseudolite positioning systems are usually deployed in areas known as “confined”. These confined areas may be buildings inside which the positioning signals transmitted by satellites in orbit around the Earth cannot be acquired because of the masking produced by the walls, the ceilings etc. They may simply be areas not covered by the satellite navigation system in question. Generally, a confined area will be defined as being an area in which positioning signals transmitted by satellites cannot be correctly acquired. On the other hand, “open areas” are spoken of in the areas in which positioning signals transmitted by satellites can be acquired by an appropriate receiver. Moreover, the satellites of which a receiver can theoretically receive positioning signals, because of the adequate relative position between said satellites and said receiver, are called “visible” to the receiver, while the other satellites of the constellation are called “not visible”. These dedicated terms, “visible” and “not visible”, can be used in the case of pseudolites, the appropriateness of the relative positions being in this case determined not by the geometry of the globe of the Earth, but by the local maskings that may affect the pseudolite signals.
The definitions given above of the terms “confined area”, “open area”, “visible” satellite or pseudolite and “non-visible” satellite or pseudolite are valid for all of the rest of the description and for the claims. European Patent Application EP 1742080 is representative of this state of the art.
A known problem that is inherent in pseudolite positioning relates to the fact that the transition from a confined area, in which the position of an object fitted with a receiver is computed by virtue of the positioning signals transmitted by pseudolites to an open area in which the positioning signals that are used are transmitted by satellites of a GNSS, must be transparent to the receiver. The known pseudolite positioning systems therefore require the receivers to constantly seek to acquire positioning signals over all of their channels. A certain number of channels is dedicated to the acquisition of positioning signals transmitted by visible satellites; these channels are associated with the spreading codes corresponding to said visible satellites. Other channels are associated with the spreading codes of the pseudolites of the confined area in which or close to which the object is found. In general, all of the available channels of the receivers are dedicated to the search for positioning signals to be acquired.
One embodiment of the present invention makes it possible to solve this problem, by making it possible to dedicate only one channel to the acquisition of signals transmitted by pseudolites.
But moreover, the main problem that is associated with pseudolite positioning systems and that the present invention makes it possible to solve is associated with interference of the positioning signals transmitted by the satellites of the GNSSs on which said pseudolite positioning systems are based.
Specifically, this problem, known as the “near-far” problem, arises from the fact that the positioning signals transmitted by the pseudolites have a power that is generally much greater than that of the signals transmitted by satellites. Thus, a user situated close to a confined area is likely to receive a positioning signal transmitted by a pseudolite of the confined area with a power of the order of 50 times that of the signals of the satellites of the GNSS that are intended to allow him to compute his position. In this case, the signal of the pseudolite “drowns out” the signals of the satellites and disrupts or prevents the correct positioning of the user. Major problems may arise, notably in the matter of safety in the civil aviation field.
In order to attempt to solve this problem, known solutions consist in optimizing the processing of the signals received by the receiver with the aid of appropriate processes within the receiver. However, these known solutions are not satisfactory because they involve increased consumption of energy and processing time for the receiver. Moreover, said receiver must be specifically adapted, which represents a considerable constraint for the users.